Physiological conditions of a subject can provide useful information about the subject's health status, such as to a physician or other caregiver. Devices may be implanted within a patient's body for monitoring certain physiological conditions. Cardiovascular monitoring or function management devices include pacemakers, cardioverters, defibrillators, cardiac resynchronization therapy (CRT), drug delivery or combinations of the above. Implantable pacers are devices capable of delivering higher energy electrical stimuli to the heart. A defibrillator is capable of delivering a high-energy electrical stimulus via leads that is sometimes referred to as a defibrillation shock. The shock interrupts a fibrillation, allowing the heart to reestablish a normal rhythm for efficient pumping of blood. These systems are able to sense cardiac signals and deliver therapy to the heart based on such signals. Additionally, the arrangement of the leads of such systems in the body allows for other physiologic signals to be sensed.
Variations in how much fluid is present in a subject's thoracic region can take various forms and can be an indication of other health concerns such as heart failure. As one example, eating salty foods can result in the retainment of excessive fluid in the thorax, which is commonly referred to as “thoracic fluid,” and elsewhere. Another source of fluid build-up in the thorax is pulmonary edema, which involves a build-up of extravascular fluid in or around the lungs.
One cause of pulmonary edema is congestive heart failure (referred to as “CHF”), which is also sometimes referred to as “chronic heart failure,” or simply as “heart failure.” CHF may be conceptualized as an enlarged weakened heart muscle. The impaired heart muscle results in poor cardiac output of blood. As a result of such poor blood circulation, blood tends to pool in blood vessels in the lungs and becomes a barrier to normal oxygen exchange. In brief, pulmonary edema may be an indicative and important condition associated with CHF.
Pulmonary edema, if it exists, may present a medical emergency that requires immediate care. While it can sometimes prove fatal, the outlook for subjects possessing pulmonary edema can be good upon early detection and prompt treatment. If left undetected (and consequently untreated), pulmonary edema may lead to death. Thus there is a need for a device and method to measure the accumulation of fluid in the lungs of a subject with early indication and low power consumption.